A variety of piezoelectric elements are increasingly used in electronic apparatuses as the electronic technology is being developed. Among such piezoelectric elements, monolithic piezoelectric elements are manufactured as below.
First, a piezoelectric ceramic raw material is prepared by a known solid-phase synthesis, and ceramic green sheets are formed by a known sheet forming technique. Then, an electroconductive pattern is formed by applying an electroconductive paste on the surface of some of the ceramic green sheets. Subsequently, the ceramic green sheet having the electroconductive pattern is disposed between the ceramic green sheets not having electroconductive patterns, followed by pressing to yield a ceramic stack. Then, the ceramic stack is fired so that the ceramic green sheets and the electroconductive paste are co-sintered to form a piezoelectric ceramic body containing the internal electrode. The piezoelectric ceramic body is provided with external electrodes to complete a monolithic piezoelectric element.
Electroconductive materials for forming the internal electrodes include noble metals, such as Pt and Pd, and base metals, such as Ni or Cu. It is desirable to use relatively inexpensive base metals from the viewpoint of material cost.
Unfortunately, in order to sinter the ceramic stack, it needs to be fired at a high temperature (for example, 1000 to 1400° C.). If the internal electrodes are formed of a base metal and the firing is performed in a normal atmosphere, the base metal may be oxidized to lose electroconductivity. Hence, the use of base metals as the electroconductive material involves performing firing in a reducing atmosphere.
The piezoelectric ceramic composition for forming the ceramic green sheets generally contains oxides. If the piezoelectric ceramic composition is exposed to a reducing atmosphere, it may turn into a semiconductor, and consequently the piezoelectric element can lose its function.
Accordingly, a piezoelectric element has been proposed which uses a piezoelectric material more resistant to reduction so that a base metal, such as Ni or Cu, is used as the material of the internal electrodes (Patent Document 1).
In Patent Document 1, the piezoelectric ceramic composition is prepared from a lead zirconate titanate (PbTiO3—PbZrO3, hereinafter referred to as PZT) compound, which is a perovskite complex oxide (general formula: ABO3), and the resistance to reduction of the piezoelectric ceramic composition is enhanced by setting the A site component content a higher than that of the stoichiometric composition and adding Ca in the A site. Patent Document 1 intends to produce a desired piezoelectric element by giving a reduction resistance to the piezoelectric ceramic composition so that the piezoelectric ceramic composition and a base metal can be fired together in a reducing atmosphere.
Other techniques using PZT compounds as the piezoelectric ceramic composition have also been proposed. For example, a monolithic electrostrictive element has been proposed which mainly uses a Ni-containing metal for the internal electrode and a PZT compound for the electrostrictive element. In the PZT compound of this electrostrictive element, Sr, Ba, or Ca is partially substituted for Pb (Patent Document 2).
In Patent Document 2, Sr, Ba, or Ca is substituted for part of the Pb of a PZT compound (piezoelectric ceramic composition) at a percentage of 20 at % or more so that the piezoelectric ceramic composition turns resistant to reduction. Consequently, the piezoelectric ceramic composition can be prevented from being reduced even if it is exposed to a reducing atmosphere. Patent Document 2 thus intends to produce a desired piezoelectric element by firing a piezoelectric ceramic composition and a base metal together in a reducing atmosphere as disclosed in Patent Document 1.
Alkali metal niobates, such as KNbO3—NaNbO3, have been known as piezoelectric ceramic compositions other than PZT compounds. For example, there have been proposed a piezoelectric ceramic prepared by adding Li2O to KNbO3—NaNbO3 (Patent Document 3), a piezoelectric ceramic prepared by adding MnO to KNbO3—NaNbO3 (Patent Document 4), and a piezoelectric ceramic prepared by adding Fe2O3 or CO2O3 to KNbO3—NaNbO3 (Patent Document 5).
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2-138781
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2-224283
Patent Document 3: Japanese Unexamined Patent Application Publication No. 48-81096
Patent Document 4: Japanese Unexamined Patent Application Publication No. 49-56198
Patent Document 5: Japanese Unexamined Patent Application Publication No. 49-100600